This invention relates to a dielectric composition. More particularly, it relates to the stabilization of an ester dielectric fluid to improve corona properties.
There are many types of dielectric liquid impregnated capacitors. These include large capacitors for high voltage use (above 660V AC) and for low voltage power use and may be referred to as energy storage capacitors, power factor correction capacitors, and high frequency capacitors. The smaller capacitors include those used in lighting and as motor run capacitors. This invention particularly relates to the last category but is not limited thereto.
One class of dielectric fluids which has enjoyed widespread use is the polychlorinated biphenyls; these have been the almost exclusive fluid used in power, motor run, and lighting capacitors. With the development of new dielectric materials, such as synthetic resin dielectrics, and the ecological questions raised concerning the use of PCBs, alternate classes of materials have been studied intensively for use as dielectric fluids.
A dielectric fluid for capacitors should have high electric strength and a relatively high dielectric constant, be capable of impregnating the dielectric of the capacitor, be easily purifiable, and have good corona properties. Good corona properties include both high corona starting and high corona extinction voltages; preferably, these voltages should be close in value. The fluid should not be too volatile or it will evaporate in processing, and it should not solidify during use.
Organic ester fluids have properties which are desirable for dielectrics. They did not find wide application because of stability problems, particularly when used in conjunction with paper. Two developments have made the use of ester fluids, particularly the phthalates, practical. The first is the recognition that hydrolysis of the ester in the presence of paper, serving as a source of water, is the major difficulty. This difficulty can be overcome by proper substitution of the ester to increase the steric hindrance about the ester carbonyl and thus prevent hydrolysis. Ross and Finkelstein have described this in U.S. Pat. No. 3,740,625, issued June 19, 1973. Further stability to ester fluids is imparted by the addition of epoxides. These materials, which have been used as scavengers for hydrogen and halogens, particularly with the chlorinated diphenyls, have been well described, e.g., by Eustance in U.S. Pat. No. 3,754,173 issued Aug. 21, 1973, U.S. Pat. No. 3,833,978 issued Sept. 10, 1974, and in U.S. Pat. No. 3,925,221 issued Dec. 9, 1975.